1. Field of the Invention
The present invention relates to an optical pickup and more particularly, to an optical pickup which can play back a compact disc and a digital video disc through two light sources.
2. Description of the Prior Art
Generally, an optical disc is widely used these days because it can record and reproduce information relatively at a high density when compared to a long play record (LP) or a magnetic recording tape and can be kept semi-permanently.
A compact disc (CD) is most widely spread and used as an optical disc. However, since the compact disc has a limited recording capacity, it cannot be reliably used for a movie, music, a game, or other multimedia which have a playing time exceeding, for example, 90 minutes. For this reason, it is necessary to prepare two or more discs and continuously play back them.
To resolve this problem, a digital video disc (DVD) is recently developed as a next generation optical disc. A DVD has a memory capacity which is 25 times that of a CD and can read ten million bit data per second. This is because the DVD has pits and tracks which correspond to halves of those of the CD. In other words, because a pit and a track of the DVD are about 0.4 feet and 0.8 micron, respectively, a high density recording is possible. Accordingly, due to this high capacity and operational capability, the DVD can be easily used in various fields such as a movie, music, a game or other multimedia.
On the other hand, while an optical pickup can be provided for exclusively playing back the DVD, it is preferred that both a CD and a DVD be played back by the same player in view of economics. Accordingly, a demand for an optical pickup which can play back a CD and a DVD, is gradually increasing among customers.
Referring to FIG. 1, there is shown a schematic view of an optical pickup of the prior art, which uses two light sources in order to play back both a CD and a DVD.
The optical pickup of the prior art includes a first light source 1 for recording and reproducing information onto and from a digital video disc D. This first light source 1 may be a laser diode which emits a beam of 650 nm wavelength.
The optical pickup also includes a second light source 2 for recording and reproducing information onto and from a compact disc Dxe2x80x2. This second light source 2 may be a laser diode which emits a beam of 780 nm wavelength. The second light source 2 is positioned such that it is parallel to the first light source 1.
A flat beam splitter 3 is arranged on a path of the beam emitted from the first light source 1, and a cubic beam splitter 4 is arranged on a path of the beam emitted from the second light source 2.
The optical pickup includes a reflecting mirror 9 for directing beams which are emitted from the first and second light sources 1 and 2 and then reflected from the flat beam splitter 3 and the cubic beam splitter 4 toward the digital video disc D and the compact disc Dxe2x80x2, and a collimator lens 5 for shaping the beams reflected from the reflecting mirror 9.
The optical pickup further includes an objective lens 6 for spotting the beams shaped by the collimator lens 5 onto the digital video disc D and the compact disc Dxe2x80x2, and a photodiode 8 for detecting recorded information and an error from the beams reflected from the digital video disc D and the compact disc Dxe2x80x2.
The optical pickup of the prior art, constructed as mentioned above, has an advantage in that stable recording and reproducing operations are ensured. That is, an aberration is generated only in an small amount during the travel of the beams emitted from the first and second light sources 1 and 2 and reflected by the flat beam splitter 3 and the cubic beam splitter 4 to be spotted onto the optical discs.
In other words, a very fine and stable spot can be obtained, as shown in a spot diagram of FIG. 2.
However, since the costly cubic beam splitter 4 must be used in order to obtain such a stable spot, expenses of the optical pickup are increased.
Also, since the first and second light sources 1 and 2 are disposed in side-by-side relationships with the flat beam splitter 3 and the cubic beam splitter 4, respectively, a relatively wide installation space is needed and it is difficult to manufacture a compact optical system.
Referring to FIG. 3, there is shown a schematic view of another optical pickup of the prior art, which uses two light sources.
The optical pickup uses an anisometrical hologram beam splitter 3a. In this respect, first and second light sources 1 and 2 are slopingly mounted such that they define predetermined angles to a plane of the hologram beam splitter 3a. 
On a path of beams which are emitted from the first and second light sources 1 and 2 while extending through the hologram beam splitter 3a, there are sequentially disposed a flat beam splitter 4a, a collimator lens 5 for shaping the beams, and an objective lens 6 for spotting the beams onto optical discs. The flat beam splitter 4a is slopingly mounted.
The optical pickup includes a photodiode 8 for detecting recorded information and an error from the beams reflected by optical discs. The photodiode 8 is disposed such that it is opposed to the flat beam splitter 4a while sandwiching the hologram beam splitter 3a therebetween.
While the optical pickup of the prior art, constructed as mentioned above, has an advantage in that without using a cubic beam splitter, expenses are reduced, it still suffers from defects in that due to wavelength variations of the beams emitted from the first and second light sources 1 and 2, a flickering of an optical axis is caused in an entire optical system.
That is, in case of a conventional laser diode, a wavelength variation through ∓20 nm is caused due to a variation in a surrounding temperature of the optical system.
Accordingly, a method for correcting a focusing error relying upon a change in a position of an optical disc by using a 4-divided photodiode, includes detecting a change of quantity of light spotted onto the 4-divided photodiode as shown in FIG. 4 and focusing the optical pickup by using the detected results to thereby correct the error.
However, in case that the anisometrical hologram is used as described above, since angles of a wave front and a wave tail are changed so that a tilt of a light axis exceeding 0.2xc2x0 is caused at an exit side of the light, a configuration of a light spotted onto the optical disc is deteriorated. At the same time, when a position of a light spotted onto the photodiode is changed relying upon a wavelength, in case that the light is focused onto the photodiode as shown in FIG. 5, the optical system cannot properly calculate a focusing error of the optical disc in terms of quantity of light which is spotted onto a portion of the 4-divided photodiode.
Namely, as shown in FIGS. 6a and 6b each of which shows an S-curve illustrating a light quantity difference in accordance with a distance between the objective lens 6 and the optical disc, a variation in quantity of the light which is spotted onto the photodiode is made non-uniform, and according to this, a quantity distribution of the light which is spotted onto the photodiode is, as shown in FIG. 7, lopsided. Hence, it is impossible to calculate through the photodiode, whereby a focusing error is caused.
Referring to FIG. 8, there is shown a schematic view of still another optical pickup of the prior art.
The optical pickup shown in FIG. 8 uses two flat beam splitters 3b and 3c. 
That is, the flat beam splitters 3b and 3c are matched with first and second light sources 1 and 2, respectively.
However, in the optical pickup of the prior art, since it is complicated to define angles between axes and the first and second light sources 1 and 2, it is difficult to construct an optical system, and an aberration property is deteriorated.
To be more detailed, as shown in FIGS. 8 and 9, an aberration is caused while beams are spotted onto optical discs by an objective lens 6 after being emitted from first and second light sources 1 and 2 and passing through the flat beam splitters 3b and 3c which are slopingly mounted at a predetermined angle.
In other words, on the basis that the flat beam splitters 3b and 3c are slopingly mounted, when assuming that xcex81, xcex82 are incident angles of the beams emitted from the first and second light sources 1 and 2, n is an index of refraction of the flat beam splitters 3b and 3c, and T is a thickness, an astigmatism can be expressed as given in an equation 1 described in below:             Equation  1:            ⁢                            Q          1                ⁢                  Q          2                    _        =                    1                  n          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            1            xe2x80x2                              ⁡              [                  1          -                                                    cos                2                            ⁢                              θ                1                                                                    cos                2                            ⁢                              θ                1                xe2x80x2                                                    ]              ⁢    T  
When a spot focused onto the optical disc is influenced by the astigmatism thus generated, it is to be readily understood from FIG. 8 that a very unstable and large spot is formed.
Accordingly, since beams emitted from the first and second light sources 1 and 2 and beams reflected from the optical discs must pass through the flat beam splitters 3b and 3c, respectively, it is difficult to construct an optical system in consideration of these facts.
Accordingly, the present invention has been made in an effort to solve the problems occurring in the prior art, and an object of the present invention is to provide an optical pickup which offsets an astigmatism generated in a flat beam splitter by light wave modulation of a hologram diffractive surface to render an optical system to be easily constructed, and does not use a costly cubic beam splitter, in order to cut down expenses.
In order to achieve the above object, according to one aspect of the present invention, there is provided an optical pickup comprising: a first light source for emitting a laser beam having a first predetermined wavelength; a second light source disposed on an optical axis such that it is substantially perpendicular to the first light source, for emitting a laser beam having a second predetermined wavelength which is different from the first predetermined wavelength; a holographic beam splitter slopingly disposed at a point where laser beams emitted from the first and second light sources are crossed with each other, the holographic beam splitter having one surface on which a hologram diffractive surface is formed and the other surface on which a coating layer for being penetrated by and reflecting the laser beams at a first predetermined ratio is formed; a flat beam splitter arranged on a first path of the laser beams of the first and second light sources which are passed through the holographic beam splitter, for being penetrated by and reflecting the laser beams at a second predetermined ratio; a collimator lens arranged on a second path of the laser beams of the first and second light sources which are passed through the flat beam splitter, for shaping the laser beams emitted from the first and second light sources; an objective lens for focusing the laser beams shaped by the collimator lens upon an optical disc; and a photodiode arranged below the flat beam splitter, for reading out recorded information and an error from the laser beams which are focused and reflected upon and from the optical disc.